American Express Card
The security code on an American Express Card is the 4 digits that are above and to the right (or left) of the embossed credit card number on the front of your card. The security code highlighted below.

Visa/Master Card
The security code on a Visa / MasterCard is the last 3 digits of signature strip on the back of your card. A Visa card is shown below with the security code highlighted.

Discover
The security code on a Discover Card is the last 3 digits of signature strip on the back of your card. A Discover card is shown below with the security code highlighted.